Abstract: Printed wiring board, PWB, signal paths must
often change layers in a board stackup. Under some conditions this can
cause problems. An ESD example is used to illustrate the conditions
where changing layers can cause problems.

Discussion: Routing paths
in a PWB often requires the paths to change layers to accomplish the
layout. For a four layer PWB, this usually means changing from the top
layer to the bottom layer of the board, the two middle layers being
power and ground. A four layer board is especially problematical
because generally the separation between the power and ground layers is
relatively large, on the order of 30 to 40 mils, compared to boards
with six or more layers.

Figure 1 shows the case of a signal path that changes from the top
layer of a four layer board to the bottom layer. While on the top
and bottom layers, the signal current is matched by its image return
current in the nearby ground or power plane. As the signal current
changes layers from top to bottom an impairment that effects ESD
performance can occur.

All signals form a loop, from source to load and back again to the source.
It's often the "back again" part of the path that gets us into trouble, as we shall
see for this specific case. The signal's returning current on the bottom of the bottom plane follows the
signal to the top of the bottom plane, but it must pass through the
interplane impedance, Z in Figure 1, to get to the bottom of the top
plane from where it can follow the signal on to the top of the top
plane.

One way to think of the impedance Z is to think of the the two planes
as a two dimensional transmission line spreading out from the signal
via. Bypass caps form low impedance "shorts" (although not all
that good of a short at high enough frequencies where their inductance
becomes
important) and the edges of the board are generally unterminated
"opens." These and other features cause reflections that contribute to
an interplane impedance that varies significantly with frequency and
can reach several Ohms at some frequencies for a four layer board with
plane spacings on the order of 30 mils. Murphy's Law has it that
one peak of this impedance will be at the third harmonic of your clock
frequency!

To evaluate this effect, I constructed the test board shown in Figure 2. Each
signal trace is about 30 cm long. The traces are composed of one
conductor of 100 Ohm twisted pair telephone wire. When taped to the
ground plane, it forms a 50 Ohm path. The board is a double copper clad
board and the whole assembly simulates a four layer PWB. The two copper
planes are spaced about 30 mils apart and are shorted together by the SMA
connectors on the left and at the load resistors on the right, four
locations. One path stays on one side while the other penetrates the
board and runs for about 10 cm on the other side.

Figure 2. Test Board with Paths on a Single Layer and Two Layers

The board was subjected to a 3 kV ESD contact discharge from an ESD simulator to the end of a one
meter cable fastened to the plane shown in Figure 2 near the middle of
the right edge while the middle of the left edge was
connected to ground to drain charge from the board. Figure 3
shows the resulting apparent signal at the SMA connector for the top
path of Figure 2 that stayed on the same side of the board. That
signal was only about 400 mV.

Figure 3. ESD Generated EMI to Path on One Layer

Figure 4 shows the apparent signal at the SMA connector of the lower
path of Figure 2 that changed layers from the top to the bottom of the board and back. In this
case, the peak signal at the SMA connector was over two volts peak and
was oscillatory at the natural frequency of the assembly. This level
would most certainly be a problem for most logic circuits. The increased noise in the lower path is due to the ESD
causing a voltage drop across the interplane impedance, Z, at each
transition from one side of the board to the other. This voltage shows
up in the signal/return loop and therefore appears at the SMA connector.

Figure 4. ESD Generated EMI to Path on Two Layers

For cases where the interplane distance is much smaller than 30 mils,
the interplane impedance will also generally be lower as well and the
effect shown in Figure 4 will be smaller and less of a problem. The
effect can also be minimized for a four layer board if critical signals
transition from top to bottom of the board near existing (for low cost)
bypass capacitors.

Summary:
Transitioning between layers of a PWB can introduce significant
impairment into a signal path. The larger the spacing between power and ground planes, the larger the
effect. The example of a "four layer" PWB response to ESD shows one of
the problems that can occur.

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